A semiconductor wafer typically comprises a plurality of polysilicon (“PC”) lines, which transport electrical signals. A cross sectional view taken from the top down of the PC line is known as the profile of the PC line. PC lines have a specific profile in a stable production line. An ideal PC profile is rectangular. Over time, however, the ideal, rectangular profile may drift or, in other words, change. For instance, a PC profile which began as rectangular may drift to trapezoidal in shape. Such drift is known as profile drift. Profile drift may be caused by any number of factors. For example, a machine failure in the wafer fabrication plant or a change in process conditions, such as a change in the Reactive Ion Etching (“RIE”) of the PC line, may cause profile drift.
There is one known technique for identification of profile drift in a PC line, namely visual detection. First, a semiconductor wafer comprising a plurality of PC lines is cut. In so doing, the cross sectional area of the PC line is revealed. Second, a photograph of the revealed cross sectional area of the PC line is taken. With the aid of a scanning electron microscope, a technician studies the photograph of the cross sectional area of the PC line. The photograph depicts the PC profile in the production line on a particular date. The technician notes the shape of the PC profile. A technician completes the same exercise on a later date. If the shape changes, the technician identifies a profile drift.
Visual detection has its drawbacks. Visual detection is a destructive technique. Once the wafer has been cut for visual inspection, the wafer cannot be further processed. Accordingly, visual detection wastes the resource of the wafer as well as needlessly consumes the time of the machines that created the destroyed wafer. In addition, visual inspection is time consuming for the technician. Accordingly, the technician does not visually detect each lot or even each bunch of lots. A technician randomly selects one wafer from one lot. Such lot is one lot out of hundreds of wafers and tens of lots. Because visual detection occurs with so few wafers, statistics cannot accurately be gathered regarding the profile drift of a given lot or even a given bunch of lots. Finally, visual detection is only as accurate as the visual faculties of the technician conducting the visual detection.
Therefore, there remains a need in the art for a structure and method of identifying profile drift of a PC line. Specifically, there is a need in the art for a standardized, nondestructive, statistically significant method for identifying profile drift.
These and other deficiencies in the prior art are overcome through the method of this invention.